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height vs diameter

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Coronafix

Fri Aug 18 2006, 09:56AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Does anyone know why the height vs diameter ratio in a tesla coil has an effect on the output?

ragnar

Fri Aug 18 2006, 10:03AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

IMHO:

#1) The height-diameter will, amongst other things, affect the length of wire and the inductance.

#2) For the highest Q, you want to maximise your stored energy (inductance/self capacitance) and minimise any losses (long thin wires, such as your resonator :P)

#3) As such, a coil where the HEIGHT is the same as the DIAMETER will enclose the maximum volume of flux for the shortest length of wire.

Please feel free to correct me. ^^

Steve Conner

Fri Aug 18 2006, 11:00AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

1) It's very difficult to get sparks more than 3-4 times your secondary's height without setting the thing on fire. So the height affects maximum spark length in that way.

2) If the secondary is too skinny, you'll find it hard to couple enough power into it.

Wilson

Fri Aug 18 2006, 01:52PM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

It seems like 1:3 to 1:5 is the accepted rule of thumb in regards to dia:height ratio. Seeing as most people use it with great success, it shouldn't be too bad

J. Aaron Holmes

Fri Aug 18 2006, 03:50PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I'm building a new lowish-power (1-2kVA) coil and going with the 3:1 aspect ratio. My reasoning is thus:

1) Making the coil really tall when it won't be putting out enormous sparks would look funny; the coil would dwarf the output.
2) Making the coil diameter small just because it's short (i.e., in order to achive a h:d ratio > 3:1) means that, in order to keep Fres low, I'd have to either:
--- 2a) Add a bunch more (and finer) turns, thereby increasing copper losses.
--- 2b) Add a really ridiculously huge topload, thereby making it look stupid (same reason as too much height).
--- 2c) Both of the above.
3) I want Fres to be low, because this lets me substitute L for C in the primary tank, thereby decreasing peak current, and thereby decreasing spark gap losses (and perhaps making quenching easier).

...so for a small coil, I'd actually prefer a 3:1 ratio, though perhaps there are reasons why this is bad? My secondary h:d is 36":11".

Regards,
Aaron, N7OE

Coronafix

Sat Aug 19 2006, 12:21AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

thanks all.

I was under the impression that coils needed to stick quite closely to aspect ratios (not 3.67:1 for example)

I understand the coupling problems of too skinny a coil and the X/R of the coil, but I got a bit confused with people always talking

about an exact ratio. But what your saying is that as long as it's somewhere between 2 & 6 :1 it's ok. Depending of course on wire

diameter, number of turns, power input, resistance.

J. Aaron Holmes

Sat Aug 19 2006, 12:51AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I'll admit, I still don't totally "get" the aspect ratio thing. At one point, John Couture espoused that "turns per inch" was a really important concept, because using too few turns, if tight-wound, would result in too great a voltage per turn and would thereby lead to insulation failure (e.g., of enamel). If this is correct, then it's tempting to think that one ought to simply consider the coil power, from which can be estimated the voltage and current in the secondary, then choose wire capable of handling the current, and add windings until your voltage per turn is below what the insulation can withstand.

At some point, presumably, those added windings (for a given coil diameter) just stop being useful, since they're so high above the primary that there's just no coupling way up there. Then you have to start growing outward (adding diameter) until those way-up turns start doing something for you. Perhaps this leads to the question: What is the minimum amount of coupling one ought to aim for at the top of the coil, and the maximum one ought to aim for at the bottom? There is an interesting "racing arcs" discussion on TCML right now that kind of ties into this.

Regards,
Aaron, N7OE

Terry Fritz

Sat Aug 19 2006, 04:21AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

If the secondary is too skinny, you'll find it hard to couple enough power into it.

Same is true if it is too "tall"...

In general, you want the secondary to be "high enough" so that arcs to the primary metal is not a big issue. For bigger coils, diameter has to increase too... 3 to 5 is "good"

I can drive lower coupling due to lower loss primary design (MMCs and short good connections). DRSSTCs and many folks like to "push" couping harder... But avoid the racing arcs that can destroy the secondary coil

Cheers,

Terry

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